The present invention relates to a technology that is directed to a method and system of evaluating the quality of a processed shape of a circuit pattern formed on a wafer using an electron beam image of the circuit pattern in the manufacture of semiconductor devices.
The present inventors have determined that the following technologies exist with regard to evaluating the quality of a processed shape of a circuit pattern formed on a wafer.
In order to obtain the desired processing performance in an etching process, for example, a supporting experiment is normally carried out in advance. The experiment is performed with a plurality of processing conditions being set as parameters, and processing conditions considered optimum are determined and registered as a recipe in the etching equipment. In this operation of optimum condition finding, the quality of etching performance is checked mainly by cross sectional observation of the pattern.
FIGS. 2A to 2B show examples showing a difference in a pattern cross section after the etching. FIGS. 2A to 2D are sectional views of gate wiring, showing examples of how this shape may change with process conditions. Generally in the gate process, shape control of a bottom part is very important. This is because the shape of the pattern bottom affects the results of a subsequent ion implantation process, and the size itself of the pattern bottom largely affects the characteristic of a device. FIG. 2A shows a shape that is generally considered most desirable, in which the slope angle of the pattern side wall is almost vertical, and flaring and other defects are not formed at the pattern bottom. On the contrary, the upward taper shown in FIG. 2B, the downward taper shown in FIG. 2C, and the flaring shown in FIG. 2D are shape abnormalities occurring from improper process conditions. It is necessary to realize the state of FIG. 2A by alteration of the processing conditions.
Next, an outline of a gate etching process and the relationship between a final shape and the processing conditions will be described with reference to FIGS. 3A to 3E. In this process, a film to be processed is subjected to etching based on a resist pattern formed in a photolithographic process. Generally, in superfine processes in recent years, it is often the case that a BARC (Bottom Anti-Reflective Coating: a reflection reducing coating at the time of exposure) is formed under the resist. FIGS. 3A to 3D are directed to such a case. Here, a case of one step of BARC etching and two steps of Poly Si etching are considered. There is also a case where BARC is processed through much more steps than this case.
After the exposure, as shown in FIG. 3A, the BARC layer is disposed on the film to be processed (in FIG. 3, Poly Si film), and the resist pattern is formed on it. In a normal production line, the size of the resist pattern is measured at this state, and whether there is an abnormality in the exposure process is checked. In the subsequent etching process, first, the BARC layer is etched (FIG. 3B). Next, Poly Si etching is performed using the resist and the pattern of the BARC film as a mask with etching conditions switched. At this time, normally the etching of the Poly Si film is processed in several sub-steps constituting the etching. First, the etching is performed vertically under conditions having relatively high anisotropy (in FIG. 3C, Poly Si etching Step 1). Next, when approaching the lower end, the conditions are switched to those having high selectivity (in FIG. 3D, Poly Si etching Step 2) so that the etching may not break through the oxide film or introduce damage, and then the etching is processed to an underlayer oxide film by making some sacrifices in terms of anisotropy. These steps of processing shown in FIGS. 3B to 3D are performed continuously by changing the conditions in a single piece of etching equipment. After the etching process, a resist removal process by ashing and washing is performed to form a gate pattern, as shown in FIG. 3E. In this series of processes, several conditions are switched. Thus, it is necessary not only to check for the presence of abnormalities, but also to determine a problem-causing step in evaluation of a processed result using sectional photographs. Condition optimization of each step is carried out by conducting, for example, the following judgments: if there is an abnormality in the slope angle of the side wall, Step 1 of the Poly Si etching is a main cause; and, if the flaring deteriorates the processed shape, Step 2 of the Poly Si etching is bad.
When the processing conditions are determined by this operation of optimum condition finding, they are registered in a recipe of the etching equipment and the etching process will be performed on the basis thereof in an actual production line. It is ideal that the etching performance at this time is exactly the same as that when the optimum condition finding is performed in advance. However, an increase/decrease of the etching rate and the like may occur due to the state of the inner wall of an etching chamber, an atmospheric change with the lapse of time, etc. Along with higher integration of the LSI in recent years, a processing performance that supports more minute processing dimensions and a higher aspect ratio is being required, and, accordingly, high-accuracy process control for a difference of a shape is desired to cope with process variations like this. At present, variation in the processed pattern shape resulting from variation of these etching conditions is detected by size measurement with a measuring SEM, or by acquiring SEM images having different slope angles and measuring its three dimensional shape by a principle of stereoscopy.
With respect to the technologies for evaluating the quality of the processed shape of a circuit pattern formed on the wafer, as described above, the present inventors have determined the following.
For example, in the conventional finding of optimum etching conditions, as described above, the quality of the processed shape is checked by cross sectional observation of the pattern. However, this check of the cross section is carried out by cleaving the wafer and observing it using a cross sectional SEM or the like, which takes a considerably long time; therefore, efficient finding of optimum conditions is rather difficult. Preparation of specimens for cross sectional observation and observation work require skills different from those used in the optimum etching condition finding and suffers from a high cost. The preparation is a destructive evaluation, so wafers after the observation need to be discarded. In order to conduct process control as well as condition finding, it is mandatory to evaluate a shape nondestructively. On the contrary, dimensional measurement by the measuring SEM makes it easy to conduct measurement nondestructively, but provides only a difference in a pattern size. Therefore, there is a problem in that information sufficient to set up the conditions of the etching process cannot be obtained.
The following description identifies problems associated with shape evaluation (size measurement) by the conventional SEM, which constitute technological problems that the present invention intends to solve.
It is common to conduct dimensional measurement by means of a measuring SEM using line profiles of secondary electron images. Thus, at first, we will review a common relationship between a cross section and a line profile of secondary electron intensity that is described in “Electron beam testing handbook”, p. 261, a material of the 98th Study Group Meeting of the 132nd Committee of Application of Charged Particle Beams to Industries, Japan Society for the Promotion of Science.
FIG. 4 shows the following:    (A) When the electron beam is irradiated onto the substrate part, the intensity of the detected secondary electron signal shows a constant value that depends on the emission efficiency of secondary electrons of the substrate material.    (B) When the beam irradiating point approaches the pattern, secondary electrons among the generated secondary electrons that collide with the slope part of the pattern increase and the capture efficiency of secondary electrons decreases, whereby the signal intensity lowers slightly.    (C) Secondary electron signal intensity exhibits a minimum in a position that shifts from the bottom edge of the pattern outward by a half of the beam diameter.    (D) After passing point C, the signal intensity increases rapidly almost linearly due to a change in secondary electron emission efficiency that corresponds to a change in the slope angle of the specimen.    (E) As the beam irradiation point approaches the top edge, the increase of the signal intensity becomes mild because each irradiation point on the slope part has a different capture efficiency of the emission secondary electrons.    (F) The secondary electron signal intensity exhibits a maximum in the position that shifts from the top edge of the pattern outward by a half of the beam diameter.    (G) The secondary electron signal intensity decreases after passing point F, and settles to a fixed value that is determined by the secondary electron emission efficiency of the pattern material.
Although FIG. 4 shows the case of a photoresist, the behavior is also the same in the case of other materials.
In order to measure the size from such a line profile, it is necessary to detect an edge position of the pattern from the line profile. As a method of detecting an edge position whose program is loaded on the measuring SEM, the following methods are known: a method of detecting a maximum slope-angle position (maximum gradient method), as shown in FIG. 5A; a threshold method of detecting an edge position using a predetermined threshold value, as shown in FIG. 5B; a line approximation method in which the edge parts and a base material part are approximated by straight lines and cross points of these lines are detected, as shown in FIG. 5C; and other methods.
However, with methods of FIG. 5A and FIG. 5B, it is impossible to know correctly which height in an actual cross section of the pattern is chosen for measurement of the size between the points determined by the height. As shown in FIGS. 2A to 2D, since the problem of the etching process is a difference in the pattern shape, a technique is needed that can make clear which height is chosen for detection of the edge positions. With a sample having such a waveform as shown in FIG. 4, it is possible to measure approximately the size of the pattern bottom part by the straight line approximation method of FIG. 5C. However, it is not necessarily possible to obtain correct measured values depending on its shape. That is, since the secondary electron signal quantity of an SEM depends on the slope angle of a pattern surface, in the case where the slope angle varies at the pattern side wall, the waveform does not become a straight line; therefore, the straight line approximation method becomes incapable of measuring correct sizes. Mere measurement of either the width of the pattern top part or the width of the bottom part cannot lead to correct evaluation of the state of the etching process. This is because, as shown in FIGS. 3A to 3E, in order to ascertain which step causes a problem, shape information corresponding to each of the steps is required. Even if a three dimensional shape inspection technique is employed that uses stereoscopy and is effective in acquiring three dimensional information, it is difficult to sufficiently obtain information useful for optimum etching condition finding. In order to perform stereoscopy, it is necessary to determine points of the image between two or more images whose beam irradiation angles are different. However, in the case where the pattern shape varies continuously and smoothly, as in the bottom part of the pattern of FIG. 3E, appropriate corresponding points cannot be obtained. This causes a problem in that sufficient evaluation cannot be performed.